Seeing galaxies through thick and thin. II. Direct measures of extinction in spiral disks through spectroscopy of overlapping galaxies

We use slit spectroscopy of overlapping pairs of galaxies to directly determine the extinction in disks of foreground spiral galaxies. The Doppler shifts of pair members are determined via cross-correlation, and their relative correlation amplitudes are used to separate their contributions to the combined spectra in regions of overlap. This spectroscopic approach is less subject to stringent symmetry constraints than our previous, purely photometric analyses. Extinctions of foreground members were obtained for six of the candidates in our sample of 18 mostly spiral/spiral pairs, when the signal-to-noise-ratio and velocity differences were suitable. In agreement with our previous imaging results, we find that the extinction in interarm regions is very modest, typically A(B) similar to 0.1 mag (corrected to face-on), while spiral arms exhibit higher extinctions of similar to0.3 mag.